Problem: On the first 3 quizzes of his history class, Luis got an average score of 86. What does he need on the next quiz to have an overall average of 88?
Let his score on the next quiz be $x$ The sum of all of his scores is then $3 \cdot 86 + x$ The same sum must also be equal to $4 \cdot 88$ Solve: $x = 4 \cdot 88 - 3 \cdot 86 = 94$.